The present invention relates to a semiconductor laser element used for optical disks, and a process for producing the same. In particular, it relates to a window-structure semiconductor laser element having excellent high-output operation characteristics, and a process for producing the same.
In recent years, various types of semiconductor lasers have been widely used as light sources for optical disc devices. Particularly, high-output semiconductor lasers are used as light sources for writing on disks in MD (mini disk) players, CD-R (recordable) drives, and so on, and a further increase in the output is strongly demanded.
One of factors that restrict an increase in the output of the semiconductor lasers is a catastrophic optical damage (COD) incidental to the increase of the optical power density in an active layer region in the vicinity of end faces of a laser resonator.
The occurrence of the COD is attributed to the fact that the active layer region in the vicinity of the end faces of the laser resonator acts as a laser light absorbing region. At the end faces of the laser resonator, there are many non-radiative recombination centers referred to as a surface state or interface state. Carriers injected into the active layer in the vicinity of the end faces of the laser resonator disappear by the non-radiative recombination. The injection carrier density of the active layer is lower in the vicinity of the end faces of the laser resonator than in the central part thereof.
As a result, regions of the active layer in the vicinity of the end faces of the laser resonator become absorbing regions with respect to the wavelength of laser light produced by the high injection carrier density of the central part of the active layer.
The higher the optical output density, the greater the local heat generation in the absorbing region. The temperature rises, and the band-gap energy (bandgap width) is reduced. As a result, there takes place a positive feedback in which the absorption coefficient further increases, which in turn causes an increase of the temperature. The temperature of the absorbing regions in the vicinity of the end faces of the laser resonator eventually reaches a melting point, which results in the occurrence of a COD.
In order to solve the problem of the COD, a technique using a window structure formed by disordering a multiquantum well-structured active layer, which is described in JP-A-9-23037, has been taken as a method of increasing the output of a semiconductor laser.
The structure of a semiconductor laser element described in JP-A-9-23037 is shown in FIG. 12, FIG. 13 and FIG. 14 as an example of a prior art semiconductor laser having such a window structure FIG. 12 is a perspective view of the semiconductor laser element, showing an end face of a laser resonator 1020. FIG. 14 is a cross sectional view taken along line XIV—XIV of FIG. 12, showing a waveguide. FIG. 13 is a cross sectional view taken along line XIII—XIII of FIG. 12 in the direction of layer thickness. In FIG. 12, reference numeral 1001 indicates a GaAs substrate, reference numeral 1002 indicates an n-type AlGaAs lower cladding layer, reference numeral 1003 indicates a quantum well active layer, 1004a indicates a p-type AlGaAs first upper cladding layer, 1004b indicates a p-type AlGaAs second upper cladding layer, and 1005 indicates a p-type GaAs contact layer. Also, reference numeral 1006 (a hatched part) indicates a vacancy diffusion region, 1007 (a hatched part) indicates a protons-injected region, 1008 indicates an n-side electrode, and 1009 indicates a p-side electrode. Also, reference numeral 1020 indicates an end face of a laser resonator, 1003a indicates a region contributing to laser oscillation in the quantum well active layer 1003, 1003b indicates a window-structured region of the quantum well active layer 1003 formed in the vicinity of the end face 1020 of the laser resonator.
Next, a process for producing the conventional semiconductor laser element will be described with reference to a flowchart shown in FIG. 15A through FIG. 15D.
On an n-type GaAs substrate 1001, an n-type AlGaAs lower cladding layer 1002, a quantum well active layer 1003, and a p-type AlGaAs first upper cladding layer 1004a are epitaxially grown in sequence (FIG. 15A).
Next, an SiO2 film 1010 is formed on a surface of the p-type AlGaAs first upper cladding layer 1004a, and the film 1010 is formed with a stripe-shaped opening 1010a extending in the lengthwise direction of a laser resonator without reaching end faces of the resonator (FIG. 15B).
Then, the wafer is annealed at a temperature of at least 800° C. in arsenic atmosphere, whereby Ga atoms are absorbed into the SiO2 film 1010 through the surface of the p-type AlGaSd first upper cladding layer 1004a which is in contact therewith, and Ga holes are produced in the p-type AlGaAs first upper cladding layer 1004a. The Ga vacancies or holes are diffused to reach the quantum well active layer 1003 in the inside of crystals, thereby disordering the quantum well structure. Since an effective width of the forbidden band is widened in the disordered active layer region, the active layer region functions as a transparent window for oscillated laser light.
Thereafter, the SiO2 film 1010 is removed and then a p-type AlGaAs second upper cladding layer 1004b, and a p-type GaAs contact layer 1005 are epitaxially grown in sequence on the p-type AlGaAs first upper cladding layer 1004a (FIG. 15C).
Next, a resist film is formed on the p-type GaAs contact layer 1005, and using a photolithography technique, the resist film is shaped into a stripe 1011 in a region corresponding to the stripe-shaped opening in the SiO2 film 1010.
Then, using as a mask this stripe-shaped resist 1011, protons are injected from the top surface side of the p-type GaAs contact layer 1005, whereby a high-resistance region 1007 that is to be a current block layer is formed.
Lastly, an n-side electrode 1008 is formed on the rear side of the GaAs substrate 1001, while a p-side electrode 1009 is formed on the p-type GaAs contact layer 1005, and then the wafer is cleaved to obtain semiconductor laser elements of the structure shown in FIG. 12.
In the conventional window-structured semiconductor laser element, as described above, the SiO2 film 1010 is formed on the top surface of the p-type AlGaAs first upper cladding layer 1004a. Then, production of Ga holes in the p-type AlGaAs first upper cladding layer 1004a, with which the SiO2 film 1010 is in contact, and Ga vacancy diffusion toward the quantum well active layer 1003 takes place. Thereby, the bandgap energy in the disordered regions formed in the vicinity of the end faces of the laser resonator is made larger than bandgap energy corresponding to the laser oscillation wavelength.
The production and diffusion of Ga vacancies or holes are supposed to take place in regions covered by the SiO2 film 1010. However, if annealing is performed at 800° C. or higher, Ga holes are produced, though a little, in the uppermost surface of a region that is not covered by the SiO2 film 1010 (an internal region of the laser resonator) due to re-evaporation of Ga atoms, and the Ga holes are diffused to the quantum well active layer 1003a. 
This will invite a variation in the peak wavelength of the photoluminescence in the internal region of the laser resonator and a deterioration in the long-term reliability due to deterioration of crystallinity of the quantum well active layer.
If the annealing temperature is lowered or the annealing time is shortened, diffusion of Ga holes to the quantum well active layer 1003a in the internal region of the laser resonator can be suppressed, but production of holes and diffusion of the holes to the quantum well active layer 1003b in the region covered by the SiO2 film 1010 become insufficient. Consequently, laser light is absorbed in the region in the vicinity of the end faces of the laser resonator.
Accordingly, the COD is liable to occur in the active layer region in the vicinity of the end faces of the laser resonator, which in turn causes a reduction in the maximum optical output in the high-power operation. Thus, sufficient long-term reliability cannot be obtained.